This invention relates to processes for preparing cracking catalysts in which the particles contain finely divided crystals of a zeolitic molecular sieve component, e.g., synthetic crystalline faujasite, and a nonzeolitic component (e.g., silica-alumina, clay, clay residue). In particular, the invention relates to such processes wherein particles containing a sodium form of the zeolite and sodium silicate undergo ion-exchange with ammonium ions to replace sodium in the zeolite by treating with a solution containing ammonium ions to effect the replacement of sodium ions with ammonium ions.
In one embodiment of the invention, the sodium silicate is present as a result of the use of a sodium silicate solution to bind fine particle size zeolitic material into attrition-resistant, fluidizable microspheres. In such processes, sodium zeolite crystals are mixed with sodium silicate solution to provide a slurry which is then spray dried to form microspheres. U.S. Pat. No. 4,520,119 is illustration of such a process. Other examples are U.S. Pat. No. 3,451,948; U.S. Pat. No. 3,515,683 and U.S. Pat. No. 3,458,454. In the process described in U.S. Pat. No. 4,520,119, the fine particle size zeolitic material is a co-product of the manufacture of cracking catalyst from calcined clay by in situ procedures such as described in U.S. Pat. No. 3,647,718 and U.S. Pat. No. 3,663,165. Use of sodium silicate to bind co-product fines of the in situ process described in U.S. Pat. No. 4,493,902 is disclosed and claimed in copending application Ser. No. 06/727,985, filed Apr. 29, 1985.
In another embodiment of the invention, sodium silicate is absorbed in particles containing the sodium form zeolite by a process referred to as silica retention. Silica retention procedures designed to reduce micropore volume of catalyst precursor particles are described in U.S. Pat. No. 4,493,902, supra at column 14, line 63 to column 15, line 21, which is incorporated herein by cross-reference. Silica retention is also described in U.S. Pat. No. 3,663,165, column 6, lines 1-21. The in situ catalyst manufacturing process described in U.S. Pat. No. 3,663,165 is similar to the process described in U.S. Pat. No. 3,647,718. In these various in situ catalyst manufacturing processes, intermediates of zeolitic microspheres obtained by in situ syntheses from preformed microspheres of calcined clay generate sodium silicate mother liquors which are either retained in the microspheres and/or a sodium silicate solution from an extraneous source is added to the microspheres.
During the manufacture of cracking catalysts by the above, as well as other procedures, it is conventional to replace at least the readily exchangeable sodium ions in a sodium form zeolite with ammonium ions. In the case of faujasite zeolite, the sodium content of the particles before ammonium exchange is roughly of the order of 14.3% by weight of the zeolitic component, expressed as Na.sub.2 O. After exchange with ammonium ions to replace nore easily exchangeable sodium, a typical sodium level is about 1.5% by weight of the zeolitic component, expressed as Na.sub.2 O. After the more readily exchangeable sodium is replaced, the particles usually undergo further ion-exchange with ions such as rare earth, ammonium or mixtures of combinations thereof. In some cases, the partially exchanged material is calcined or steamed to facilitate release of difficult-to-exchange cations. This is followed by further exchange, typically with ammonium ions, rare earth ions, or mixtures or combinations thereof. Generally, sodium content (Na.sub.2 O) of the finished catalyst is less than 1 wt. %, frequently less than 0.5% wt. %, based on the total catalyst weight.
Solutions of sulfate chloride and nitrate salts of ammonium are commonly used to ion-exchange sodium in zeolites intended for use as cracking catalysts. Typically, exchange is with ammonium nitrate at pH 3-5 and elevated temperature, close to the boiling point. See, for example, U.S. Pat. No. 4,493,902 and U.S. Pat. No. 4,520,119. The '119 patent discloses (column 9) . . . "(I)t is desirable to maintain a pH low enough to ensure gellation of the silica but not destroy the zeolite content of the microspheres." The '119 patent teaches that optimally, the pH should be in the range of 3-5. In U.S. Pat. No. 3,451,948 (supra) ion-exchange after spray drying a slurry of fine particle size zeolitic material in sodium silicate solution is conducted by neutralizing the slurry at pH 5.5 at 30.degree.-40.degree. C. in the absence of ammonium nitrate, followed by exchange at pH 5.5-8.5 at 30.degree.-40.degree. C. with ammonium nitrate. The '948 teaches that pH should be maintained above 4. At alkaline pH values, ammonia fumes may cause difficulties in commercial operation. At strongly acidic pH values, destruction of the zeolite crystals may occur.
The commercial value of zeolitic cracking catalysts is influenced by activity, selectivity and attrition-resistance. Activity and selectivity are influenced by sodium level. Generally, low sodium is a prerequisite for the high activity desired in present FCC (fluid catalytic cracking) units. Attrition-resistance is dependent on manufacturing procedures. In the case of procedures that employ silica-retention or binding particulate zeolite with sodium silicate, variations in ion-exchange procedures can affect attrition resistance, activity and/or selectivity. Difficulty may be experienced in reducing sodium content to very low levels and maintenance of cracking activity may be therefore be less than desired.